This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
In the process of electrostatographic printing, an electrostatic latent charge pattern is recorded and reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography is a class of electrostatography which employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent charge pattern. Electrography is that class of electrostatography which utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent charge pattern. Development, which is the act of rendering an electrostatic latent pattern or image viewable, is employed in all of the aforementioned classes of electrostatography. Hereinafter, an electrophotographic printing machine will be described as an illustrative example of this process.
An electrophotographic printing machine utilizes a photosensitive element having a photoconductive insulating layer charged to a substantially uniform potential so as to sensitize its surface. The charged photoconductive surface is exposed to a light image of an original document being reproduced. As a consequence of this exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the surface. In this manner, an electrostatic latent image is recorded on the photoconductive surface corresponding to the original document. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing the latent image into contact with a developer mix. Typical developer mixes are well known in the art and generally include dyed or colored heat settable thermoplastic particles known as toner particles which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive member. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to move from the carrier granules to the electrostatic latent image. This concept was originally disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, and is further amplified and described by many related patents in the art.
Many factors influence the quality of the developed image, the most significant factor being the uniformity with which the toner particles are deposited on the electrostatic latent image recorded on the photoconductive member. Heretofore, development systems have employed rotary impellors, bucket conveyors, and magnetic brush systems to achieve the requisite uniformity in toner deposition. The magnetic brush system achieves a high degree of uniform toner deposition, and, therefore, is utilized in numerous electrostatographic printing machines. Generally, a magnetic brush development system includes a developer roll having a directional flux field for bringing the magnetizable developer mix into contact with the electrostatic latent image recorded on the photoconductive member.
Multi-color electrostatographic printing produces a series of electrostatic latent images corresponding to a single color of the original document. Successive partial color light images are employed to record each of the single color latent images. Each single color electrostatic latent image is developed with toner particles of a color complimentary to the color of the light image to form a subtractive system.
Generally, a multi-color development system utilizes a plurality of developer rolls, each being adapted to furnish the appropriate color toner particles to the photoconductive member. This requires that only one development unit be closely adjacent to the electrostatic latent image with the remaining units being spaced therefrom. Thus, successive electrostatic latent images are developed with different color toner particles.
The developer mix is advanced from a sump in the developer housing to the developer roll by a paddle wheel. Heretofore, the paddle wheel including a plurality of buckets secured to the periphery of a cylindrical member. The side walls of the buckets were of an equal height. Rotation of the cylindrical member caused the buckets to fill with the developer mix and advance in an upwardly direction to the developer roll. The foregoing type of system is described in U.S. Pat. No. 3,854,449 issued to Davidson in 1974. However, in systems of this type, the toner particles frequently bridge across the top of the buckets resulting in an uneven amount of material being advanced by each bucket. This introduces significant problems in eveness of toner deposition and may degradate the performance of the system. Moreover, in a system of this type, wherein additional toner particles are furnished to the developer mix to maintain the concentration thereof substantially constant, erroneous readings may be noted. It is apparent that if insufficient developer mix and toner particles are advanced to the photoconductive member, the monitoring system will indicate that the toner particles concentration level is low. Accordingly, excessive toner particles may be furnished to the developer mix increasing the concentration above the acceptable level.
Accordingly, it is a primary object of the present invention to improve the development of an electrophotographic printing machine by advancing the requisite quantity of developer mix to the electrostatic latent image recorded on the photoconductive member.